In general, an image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor may be roughly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
A CIS includes a photodiode and a MOS transistor formed in a unit pixel, and obtains an image by sequentially detecting electrical signals of unit pixels in a switching manner.
In a related art CIS structure, a photodiode and a transistor are horizontally arranged.
Although the related art horizontal type CIS has solved certain limitations of CCD image sensors, it still has several problems.
In a horizontal type CIS according to the related art, a photodiode and a transistor are horizontally formed adjacent to each other on a substrate. Therefore, an additional region for forming the photodiode is required, which may decrease the fill factor and limit the possibility of resolution.
Also, in the horizontal type CIS according to the related art, it is very difficult to achieve the optimized process of concurrently forming the photodiode and the transistor. That is, although a shallow junction is required for low sheet resistance in a rapid transistor process, it is not appropriate in a photodiode.
Furthermore, in a horizontal type CIS according to the related art, as additional on-chip functions are added to the image sensor, the size of a unit pixel must increase in order to maintain the sensitivity of the image sensor or the photodiode area must decrease in order to maintain the pixel size. However, the increase of the pixel size reduces the resolution of the image sensor, and the decrease of the photodiode area reduces the sensitivity of the image sensor.